1. Field of the Invention
The present invention relates generally to a red phosphor for a fluorescent display and a preparation method thereof, and in particular, to an SrTiO3-based red phosphor for a fluorescent display having high luminescence efficiency and high color purity and a preparation method thereof.
2. Description of the Related Art
A fluorescent display, especially, a field emission display (FED) is a flat panel display operating on the same principle of a Cathode-Ray Tube(CRT). The fluorescent display includes a cathode plate that is a field emitter array (FEA) panel for emitting electrons caused by an electric field, instead of thermal electrons; and an anode plate that is a fluorescent panel for emitting lights with the electrons emitted from the cathode plate. The cathode plate and the anode plate are separated from each other at a predetermined distance and packaged together under high vacuum. For the existing CRT tube, sulfide-based phosphors having high color purity and high luminescence efficiency are generally used. However, the FED generally operates at a low voltage of less than 5 kV, since the distance between the cathode plate and the anode plate is so short that there occurs discharging at a high voltage of above 10 kV as in the CRT tube. Therefore, many studies have been made on development of FEDs workable at a low voltage of less than 1 kV.
In the case where electrons have a low energy of 1 kV or less, electrons can be injected as deep as at most 20 nm from the surface of the phosphor so that the phosphor for the low-voltage FED has luminescence much lower than the CRT tube operating at a high voltage. Therefore, the surface condition of the phosphor greatly affects the luminescence efficiency of the phosphor. Specifically, the existing sulfide-based red phosphor widely used in the CRT tube, Y2O2S:Eu provides low luminescence efficiency and low color purity at a low voltage for the FED fluorescent display. Also, irradiation an electron beam on the surface of the sulfide-based phosphor for a long time causes a small quantity of sulfur to be released from the -sulfide-based phosphor. This leads to degrade vacuum in a small gap of about 1 mm between the cathode plate and the anode plate as in the FED panel or damage the FEA, thus deteriorating performance of the display.
To solve this problem, there has been an attempt to prepare a red phosphor for a low-voltage FED using Y2O3:Eu that is an oxide-based phosphor not ready to release sulfur, or surface-treated Y2O2S:Eu. The related methods are well disclosed in the following documents.
There is disclosed in U.S. Pat. No. 5,525,259 a method of preparing a Y2O3:Eu lamp phosphor with high resistance to water by firing a mixture of yttria, europium oxide and a flux, pulverizing the fired mixture to have an average particle size of greater than 5 measured as a Couter counter value, and coating the surface of the Y2O3:Eu particles thus obtained with gamma alumina by a chemical vapor deposition method.
K. G. Cho et al, xe2x80x9cImproved Luminescent properties of pulsed laser deposited Eu:Y2O3: thin filmxe2x80x9d, Appl. Phys. Lett., 71(23), 3335-3337, 1997 discloses a method of preparing a red thin film phosphor by depositing an europium-activated yttria (Eu:Y2O3) fluorescent thin film on a diamond-deposited silicon wafer using a pulse laser deposition method.
However, the phosphor preparation method involving Y2O3:Eu surface treatment has such a complicated procedure increasing a cost and the Y2O3:Eu phosphor has a limitation in use as a red phosphor for a low-voltage FED due to its low luminescence efficiency and low color purity at a low voltage.
It is, therefore, an object of the present invention to provide a red phosphor for a fluorescent display with high luminescence efficiency and high color purity at a low voltage without releasing any gases after a long-term projection of electrons.
It is another object of the present invention to provide a method of preparing the red phosphor for a fluorescent display in a simple and economical way.
To achieve the above objects, there is provided a phosphor for a fluorescent display that includes at least one divalent transition metal and at least two trivalent metal added to SrTiO3 and having a formula of:
SrTi1xe2x88x92xxe2x88x92yMxNyO3:zPr3+
wherein M represents the divalent transition metal selected from the group consisting of Zn, Mn, Co, Ni, Cu and Cd, N represents the trivalent metal selected from the group consisting of Ga, Al, In and B, and 0xe2x89xa6xxe2x89xa60.1, 0xe2x89xa6yxe2x89xa60.1 and 0xe2x89xa6zxe2x89xa60.1.
There is also provided a preparation method of a phosphor for a fluorescent display having a formula of:
SrTi1xe2x88x92xxe2x88x92yMxNyO3:zPr3+
wherein M represents a divalent transition metal selected from the group consisting of Zn, Mn, Co, Ni, Cu and Cd, N represents a trivalent metal selected from the group consisting of Ga, Al, In and B, and 0xe2x89xa6xxe2x89xa60.1, 0xe2x89xa6yxe2x89xa60.1 and 0xe2x89xa6zxe2x89xa60.1. The preparation method includes the steps of: adding a specified chemical agents -strontium oxide, titanium oxide, at least one divalent transition metal selected from the group consisting of Zn, Mn, Co, Ni, Cu and Cd, and oxide of at least one trivalent metal selected from the group consisting of Ga, Al, In and B, praseodymium (Pr) oxide or praseodymium salt, and mixing the resulting material homogeneously; and firing the mixture.
Preferably, the strontium oxide is SrCO3 or SrO, the titanium oxide is TiO2, the divalent transition metal is Zn, the trivalent metal is Ga and the praseodymium salt is PrCl3. xH2O.
In addition, it is preferable that the mixture is fired at a temperature of 900 to 1,300xc2x0 C. for 10 hours or less. This makes it possible to optimize the surface composition, the particle size and the shape of the final product, phosphor powder, thus maximizing the optical characteristics of the SrTiO3xe2x88x92 based phosphor.
In accordance with the present invention as stated above, the structure of SrTiO3 is modified by the addition of Ga3+ and Zn2+ as dopants to SrTiO3 which was prepared from SrCO3 or SrO, and TiO2. Modification of structure leads to enhance excitation efficiency of an activator, Pr3+, and its luminescence increased. And also it has a single emission band and thus high color purity. That is, the present invention involves addition of Ga3+ and Zn2+ instead of the composition ratio of SrCO3 or SrO to TiO2 at 1:1. The additives replace some part of Ti at the B site in the ABO3 perovskite structure with Ga3+ and Zn2+ and thus modified the micro-structure of product. This modification leads to increase excitation efficiency of Pr3+, thereby enhancing luminescence and color purity of the phosphor.